Effect of ocean internal waves on the interference component of the acoustic field in the Long-range Ocean Acoustic Propagation Experiment

Abstract

The propagation of energy along the sound‐channel axis cannot be described in terms of geometrical acoustics because of the presence of cusped caustics repeatedly along the axis. In neighborhoods of these cusped caustics a very complicated interference pattern is observed. Neighborhoods of interference grow with range and at long ranges they overlap. This results in the formation of a complex interference wave‐the axial wave‐that propagates along the sound‐channel axis like a wave belonging to a crescendo of near‐axial arrivals. In this paper, the axial wave is simulated for the LOAPEX CTD data measured at seven different ranges from the vertical line array. A signal with the center frequency of 75 Hz and 37.5‐Hz bandwidth is used for computations. This signal well approximates one transmitted m‐sequence in the LOAPEX experiment. The effect of environmental variability, induced by internal waves, on the axial wave is studied. The sound‐speed fluctuations caused by ocean internal waves are obtained with the use of the buoyancy frequency profile measured in the LOAPEX. Calculations are based on the integral representation of the axial wave in a local coordinate system introduced in the vicinity of the range‐variable sound‐channel axis. [Work supported by ONR.]